In T.D.M.A. satellite telecommunications, there are several types of system available, and some of them have been used for several years, for example the U.S. system SBS.
Other systems which have recently been put into service include, in particular, the French TELECOM 1 system which is an intra-company system, and the T.D.M.A. INTELSAT system which is used for international telephone and data transmission.
Some of these systems, for example the TELECOM 1 satellite system, use a so-called "demand assignment" system on a per call basis, as described, for example, in an article by J. C. Bousquet "Time division multiple access system with demand assignment for intra-company network using the satellite TELECOM "1" which appeared in "5th International Conference on Digital Satellite Telecommunications, Genoa, 23-26 Mar. 1981" at pages 11 to 18.
The function of a demand assignment system is to define the exact position where a newly arriving call is to be located in the transmission frame, with the call being transmitted by means of a burst whose length depends on the data rate to be transmitted, on the TDMA data rate, and on the length of a frame. So long as no rearrangement takes place, the time position of such a burst location remains unaltered from one frame to the next. Such a system must therefore be capable of indicating which burst is to receive a new call, the position of the call within the burst, and whether the burst needs to be modified in order to take the call, i.e. usually whether the burst needs to be lengthened. There also exist special cases, such as a frame in which no burst has yet been created, or in which there is no burst capable of taking the call, in which case the demand assignment system must be capable of giving all the information required to the network to enable a suitable new burst to be established.
When the frame is practically empty, there is no problem in placing a new burst. However, in the frequently occurring case of a frame which is already occupied by a relatively large number of other bursts, it becomes necessary to find a location which is large enough for conveying the new call, and this may require not only the operation of creating a new burst, but also the operation of displacing one or more other bursts within the frame in order to open up sufficient space for the new burst.
By definition, in TDMA each burst within a given frame is emitted by a different station. If it is desired, as in TELECOM 1 which uses a demand assignment system, to open up sufficient room within a frame to allow a new burst to be created, it becomes necessary to transmit a considerable amount of information to several stations, and this requires a fairly complex network.
The demand assignment system to which the present invention applies is not low transmission rate data traffic which can be transmitted and switched on a per burst basis with each burst containing complete routing information (i.e. the present invention is not concerned with so-called "bursty" traffic). Some known systems, e.g. ALOHA-SLOTTED ALOHA, use random transmission channel seizing, with full repetition of the transmitted data in the event of a collision.
In contrast, the present invention relates to calls set up on a call-by-call basis, e.g. ordinary telephone calls, for which all the characteristics defining the call (e.g. the locations and the sizes of the TDMA bursts) are defined at the beginning of the call so that once this data is available to all of the stations concerned there is no longer any risk of collision. Such a call requires "clearing down" at the end thereof, and the corresponding bursts are then no longer transmitted.
A first aim of the present invention is to provide a method and a system for demand assignment which require hardly any burst rearrangement within a frame, thereby considerably simplifying the system.
Furthermore, the complexity of this type of conventional demand assignment system, although large is not unmanagable when transmitting telephone channels at 64 kilobits per second, for example, since each channel occupies relatively little room within a frame, however the complexity considerably increases and becomes practically inextricable when video channels are to be transmitted in a frame (e.g. TV or video conferences) since the data rate of such channels may lie in the range 2 megabits/second to 30 megabits/second.
There is high demand at present for satellite transmission of television channels, and more generally for all kinds of image transmission. This traffic is expected to increase considerably over the next twenty years, both concerning international TV transmission and concerning video conferences within a region or between regions.
Another aim of the invention is to provide a method and a system for demand assignment which provides simple means for inserting video telecommunications channels in an existing frame.